N7 back calculation results

The N7 FWD results were used to back calculate stiffness values for the different layers in the pavement structure. The method specified in Section 3.4.2 was used during the back calculation of the N7 FWD results. These stiffness values were analysed and used to determine the stress conditions induced by loading as discussed in Section5.1. This section discusses the results obtained from back calculating layer stiffnesses and focuses on the BSM resilient modulus development over time.

FWD test results for the N7 were available for 1994, 2000 2004 2010 and 2016. The available information for each of these points in time included the location of the test, surface temperature, air temperature, month of testing, FWD load and deflection measurements. The FWD load and pressure induced was not measured for each blow, but the load was given as 40 kN. The layer thicknesses used during back calculation of the N7 FWD results were based on observations from test pits and trenches dug on the slow lanes and are shown in Tables O.1 and O.2. The Poisson’s

ratio for granular materials and BSMs was chosen as 0.35.

Each of the FWD results were analysed and layer stiffnesses were back calculated. Due to the large amount of data obtained from the back calculation, the data was summarized for this report. The results of the back calculation for each point at each of the points in time can be found on the attached CD. The average back calculated layer stiffnesses, obtained from the 2016 FWD results, for each of the uniform sections, are shown in Tables 4.4to4.7.

Table 4.4: Summary of back calculated layer stiffnesses for the N7 Southbound uniform sections in 2004

Table 4.5: Summary of back calculated layer stiffnesses for the N7 Southbound uniform sections in 2010

The back calculation results for the foamed BSM sections of the N7 were available for 2004, 2010 and 2016. Table 4.4 shows the back calculated layer stiffness for each of the uniform sections on the Southbound carriageway in 2004. Table 4.5 shows the stiffnesses for 2010 and Table 4.6 for 2016. Note that in some cases the resilient modulus of the subgrade (layer 5) is much higher than for a typical subgrade. This was a result of the focus placed on the base deflection bowl at depths between 0 and 300 mm.

Table 4.6: Summary of back calculated layer stiffnesses for the N7 Southbound uniform sections in 2016

Figure 4.20: N7 Southbound back calculated resilient modulus development in BSM layers per uniform section

The back calculated stiffnesses obtained for the BSM base were further investigated. Figure 4.20

shows the average back calculated resilient modulus development over time for the each of the uniform sections identified on the N7 Southbound carriageway. This allowed investigation of the long-term stiffness behaviour of each of the sections. The back calculated stiffnesses for the three points in time were compared for and between uniform sections.

Uniform sections 1, 2, 3 and 5 show an high initial resilient modulus at two years after construction. The average resilient modulus reduced for these four sections from 2004 and 2010 . This reduction may be due to damage to the materials due to traffic. The resilient modulus was observed to

increase between 2010 to 2016, with the average resilient modulus higher in 2016 than in 2004 for some of the sections.

Uniform section 5 showed a gradual decrease in resilient modulus from 2004 to 2016. In contrast, uniform section 6 and 8 showed a significant increase in resilient modulus over the same period. The long-term resilient modulus development varies significantly between the uniform sections. With limited information available for the local conditions, explanations and conclusions based on these trends may be unreliable.

The dotted red lines in this figure indicates the maximum allowed resilient modulus limit for BSM design as per SAPEM(2014). The higher resilient modulus limit indicates the maximum resilient modulus for a BSM 1, while the lower value is the limit for a BSM 2. These limits were obtained from the PN design method and would therefore be conservative. It would therefore be expected that the resilient modulus of the BSM in the N7 would exceed these values. However, while most of the average long-term stiffnesses were found to exceed 600 MPa, the values shown in Figure4.20

are very high.

The stiffness values were further investigated by determining the 10th and 90th percentile resilient modulus values for each section. The variation in resilient modulus for each uniform section on the Southbound carriageway is illustrated in Figure 4.21. From this figure it is clear that the back calculated stiffness values are much higher than the design values specified in the TG2. In three of the uniform sections (2, 4 and 6) very high resilient modulus values were observed. The average back calculated resilient modulus for each of these sections was in excess of 1000 MPa. The overall average resilient modulus was above 600 MPa for all but one uniform section.

These very high resilient modulus values suggest that the BSM may have been over cemented and is behaving more like a cemented material than a BSM. Field investigations confirmed this suspicion as pockets of highly cemented materials were found in the base layer. The pavement also showed cracking in some areas, which is the typical failure mechanism of cement stabilised materials. Therefore, care was taken with the N7 results as they may not be representative of a typical BSM.

Figure 4.21: N7 Southbound back calculated resilient modulus per uniform section

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The FWD data from the Northbound carriageway of the N7 highway was used to back calculate layer stiffnesses. Stiffness values were obtained for the different layers in 2004, 2010 and 2016. The average back calculated layer stiffnesses are shown in Table 4.7. Note that only uniform section 1’s stiffnesses were included, as the BSM structure for uniform sections 2 and 3 were only constructed in 2007. Similar to the Southbound carriageway, resilient modulus values obtained for the selected subgrade and the subgrade were found to be unrealistic in some cases. Values representative of the type of materials were selected for these layers and used for further analysis.

Table 4.7: Summary of back calculated layer stiffnesses for the N7 Northbound uniform sections

The back calculated stiffness results obtained for the BSM base layer were further investigated. Figure 4.22shows the average resilient modulus values back calculated for each of the three points in time per uniform section. These resilient modulus values were also found to be higher than the maximum resilient modulus specified for design purposes. As the design limits are conservatively low, the results were expected to exceed this value (600 MPa).

Uniform section 1 was selected as the whole Northbound foam section between km 5.8 and km 11.2. The trend in back calculated stiffness values indicates that the resilient modulus of the BSM was very high shortly after construction. The average stiffness value reduced from around 1200 MPa to less than 700 MPa in measurements taken between 2004 and 2010. This reduction may be due to damage accumulated due to repeated loading induced by traffic. The average back calculated resilient modulus value increased slightly between 2010 and 2016 despite the damage caused by the traffic loading.

Uniform sections 2 and 3 were part of the emulsion stabilised section which was constructed in 2007. Therefore these two sections only have two points in time where the FWD testing was conducted on the BSM. From Figure 4.22 it is clear that the BSMs in these two sections had a very stiff response. The development of resilient modulus for these two sections showed a significant increase between 2010 and 2016. The average back calculated resilient modulus values was above 1200 MPa for both of these uniform section in 2016 for both of these uniform sections. These high resilient modulus values may indicate excessive cement in the BSM. Care was taken when analysing this material as it may not be representative of a typical BSM.

Figure 4.22: N7 Northbound back calculated resilient modulus development per uniform section

The variance of back calculated stiffness results was investigated for the Northbound carriageway. The results of the 90th and 10th percentile analysis are illustrated in Figure 4.23. The resilient modulus values used for this figure were based on the back calculation of FWD results from the 2016 tests. It can therefore be assumed that this analysis reflects the current condition of the uniform sections.

Uniform section 1 in Figure 4.23 shows resilient modulus values typical of a BSM. The average resilient modulus for this section was slightly below 800 MPa, while the stiffest values were below 1200 MPa. From this figure it is clear that the BSM in uniform section 1 has a much lower resilient modulus than the two uniform sections implementing the emulsion stabilised base. Back calculated

resilient modulus values for the Northbound carriageway did not show significant variation and were deemed to be more uniform than those of the Southbound carriageway.

Figure 4.23: N7 Northbound back calculated resilient modulus per uniform section

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